This invention relates to obtaining geo-location estimates and, more particularly, for improving accuracy in obtaining such estimates in mobile communications networks. The invention also relates to combating near-far interference in wireless communication environments.
In mobile communications networks, for example, Universal Mobile Telecommunications Systems (UMTS) employing Frequency Division Duplex (FDD), base stations continually broadcast pilot signals that are spread using a known (standardized) pseudo-random sequence. All base stations in a UMTS network use the same pilot signal sequence. However, pilot signal sequences used by base stations are offset from one another in time. UMTS mobile wireless terminals (and wireless terminals in other multiple access systems that may employ Code Division Multiple Access (CDMA) based technologies or other spread spectrum technologies) are capable of measuring the relative phase differences between any two detected pilot signal sequences. This measurement capability provides a mechanism that can be used to help determine the location of a mobile wireless terminal.
Mobile wireless terminal estimates of the relative phase difference of detectable pilot signals are used by the Observed Time Difference of Arrival (OTDOA) geo-location technique outlined in the UMTS system specification (see for example, 3PP TS 25.305 v5.4.0, xe2x80x9cStage 2 functional specification of User Equipment (UE) positioning in UTRANxe2x80x9d). One such prior art system is shown in FIG. 1.
It is noted without further comment that circle 105 is based on the round trip time estimate of a transmission from mobile wireless terminal 101 to serving base station 102 and back; that hyperbola 106 is the OTDOA hyperbola along which a pilot signal phase measurement is constant between serving base station 102 and neighboring base station 104; and that hyperbola 107 is the OTDOA hyperbola along which a pilot signal phase measurement is constant between serving base station 102 and neighboring base station 103.
Pilot signals from at least three different base stations 102, 103 and 104 are required to accurately estimate the position of a mobile wireless terminal 101 using the OTDOA method, or any other geo-location method employing triangulation of pilot signal measurements of the serving base station 102 and neighboring base stations 103 and 104. Because of the interference-limited nature of mobile communications systems and, in particular, CDMA based systems such as UMTS, detecting pilot signals from two or more neighboring base stations is often not possible over a large portion of the coverage area of a cell. It is well known, for example, that due to the near-far effect, when all base stations are transmitting at maximum power, only the pilot signal from the serving base station 102 can be detectable by the mobile wireless terminal 101 over as much as 40% of the cell""s coverage area (the region nearest the base station). When only a single base station is detectable in these prior known systems, the error of the geo-location estimate is unacceptably high. This error can cause OTDOA-based algorithms to fail to satisfy the stringent position error requirements outlined by the United States Federal Communications Commission (FCC) in the Phase II of the Enhanced 911 (E911) Mandate for emergency calls (see for example, FCC E911 Calls, www.fcc.gov/911/enhanced):
for network-based solutions: within 100 m for 67% of calls and within 300 m for 95% of calls;
for handset-based solutions: within 50 m for 67% of calls, and within 150 m for 95% of calls.
Visibility of more than one pilot signal is a serious issue, especially in the FDD system of UMTS, from a geo-location perspective. This is because the transmission from the strongest pilot signal, the one creating the near-far interference, the phenomena that does not allow to measure other pilots, is continuous.
For clarity of this invention, the global interference level at a mobile wireless terminal in a wireless communication environment is generated by all non-desired signals that generate activity, voluntarily or involuntarily, on the same bandwidth with the signal coming from the desired communication unit. Referring to the scenario addressed by this invention, the desired signal to the mobile wireless terminal is (are) the one (those) coming from the serving base station(s). Note than in CDMA, the soft handover feature allows the mobile wireless terminal to communicate with more than one base station simultaneously, thus mobile wireless terminal has multiple serving base stations in soft handover scenarios. The non-desired signals are those signals coming from other base stations that are not serving the mobile wireless terminal. Due to the reuse factor of one (1) often employed by the systems based on spread spectrum technologies such as CDMA, such systems are inherently characterized by both intra-cell interference and inter-cell interference. The intra-cell interference comes from concurrent communications that are active on the cell area covered by the serving base station(s). The inter-cell interference comes from concurrent communications that are active on neighboring cells. The effect of intra-cell and inter-cell interference is cumulative, and it is what ultimately the mobile wireless terminal measures in order to monitor the system conditions for quality link control for example. The applicant has observed that there is a link between the global level of interference that is present at the mobile wireless terminal location and the visibility of pilot signals from different base stations at the same mobile wireless terminal location, thus the expected accuracy of the geo-location process. This aspect will be further exploited in this invention
It can be shown that the global interference has the highest values when close to the serving base station 102, and not in the region of borders of adjacent cells. Actually, the global interference level decreases when the mobile wireless terminal 101 is moving away from the serving base station(s), e.g., for example base station 102 shown in FIG. 1. The individual contribution of both intra and inter-cell interference to the global interference is that the level of intra-cell interference decreases with the distance from the base station and the behavior of the inter-cell interference is exactly opposite (see for example, D. Calin, xe2x80x9cGeo-location Issues and Accuracy Performance in Wireless Networksxe2x80x9d, invited paper to the ASWN02 (Applications and Services in Wireless Networks) workshop, July 2002, France).
The high level of global interference in the region close to the serving base station makes possible reception from only one pilot signal, the one from the serving base station. The unavailability of other reference, i.e., pilot, signals may affect dramatically the geo-location accuracy. For all the situations where only one pilot signal is available, the mobile wireless terminal can be located anywhere in the serving sector of a cell at a distance given by the round trip delay. This is a problem in any system based on the FDD system of CDMA or any other wireless technology where the transmission of pilot signals is continuous.
The pilot signal visibility depends dramatically on the system loading, and if the system is operated at heavy loading, there is a very high probability to measure only one pilot signal. The near-far interference region is very large in these scenarios and this affects dramatically the mobile positioning accuracy.
One approach to increasing the possibility of mobile terminals detecting two or more pilot signals from neighboring base stations is the introduction of the Idle Period in DownLink (IPDL) feature in the UMTS system specification. IPDL is currently an optional feature of UMTS networks (see the xe2x80x9cStage 2 functional specification of User Equipment (UE) positioning in UTRANxe2x80x9d noted above). The IPDL solution decreases system-wide interference by temporarily switching off the serving pilot signal for a period of time. This solution has significant drawbacks, however. Temporary muting of pilot signals adversely affects the performance of the downlink channel for on-going calls, increasing frame error rates, thus risking to degrade calls quality, which may lead to undesirable call interruptions. Furthermore, implementing IPDL requires architectural changes to the UMTS network and complicates critical functions such as downlink power control.
Problems and limitations of prior known geo-location arrangements for use in mobile communications networks are overcome, in accordance with the instant invention, by employing both FDD and Time Division Duplex (TDD) modes of operation, and is applicable when both FDD and TDD systems are jointly deployed or both FDD and TDD modes of a wireless system such as UMTS are jointly activated.
Applicant has determined that the near-far interference can be much easier controlled on purpose in the TDD mode than in the FDD mode thanks to the slotted data transmission nature of a TDD system that permits adjustment of the interference level at the time slot level instead of the frame level. Indeed, in a TDD system, loading and interference levels can be gracefully decreased in a single time slot without harming the ongoing traffic or complicating the power control operations, as would be the situation in a UMTS FDD system via IPDL or similar solutions that require to mute the pilot transmission. Applicant""s invention applies to both situations whether or not the terminals are equipped with a dual receiver.
This is realized by detecting the level of interference in the FDD system and comparing it with a first threshold. If the interference level is greater than the first threshold, operation is switched to the TDD system to perform the geo-location process. Otherwise, the geo-location process is performed in the FDD system. However, when in the FDD system, if the accuracy of the geo-location measurement is not determined to be satisfactory because, e.g., an insufficient number of pilot signals can be measured in a predetermined time interval, the operation is also switched to the TDD system to perform the geo-location process. Then, when in the TDD system, a time slot is selected that has low interference for the purpose of signal measurements. Once the time slot is selected the interference level is reduced by employing a prescribed technique until the interference level is less than a second threshold. Thereafter, the geo-location process is effected in the TDD system.